Module now requires PHP version 5.3

Thanks for this nice piece of work and it's really helpful for my backup routing.
With the latest DEV release, users with PHP versions prior to 5.3 will receive a fatal error because anonymous functions are supported in PHP 5.3 and later.

Line 135 of destinations.dropbox.inc
spl_autoload_register(function($class) {

I think we should now have this requirement in the backup_migrate_dropbox.info file so users will not face to this issue.
php = 5.3

I'd happily provide a tiny patch but not sure what you'd say.

Thanks!